The present invention relates generally to the loudspeaker field and, more particularly, to a defined-coverage loudspeaker horn.
Early systems for directing sound over a predefined area typically involved a number of cone-type loudspeakers grouped together, as in linear, two-dimensional and phased arrays. However, such systems were only modestly successful at distributing high frequency sound. They were also costly, particularly when the area was large or irregularly shaped.
Horns were first introduced to increase the efficiency at which sound is produced in an audio system. Efficiency was of primary concern because amplifiers were very costly and limited in output. However, recent advances in amplification systems have shifted the emphasis from efficiency to considerations of coverage, directivity and frequency response. Two horns addressing these considerations are disclosed in U.S. Pat. No. 2,537,141 to Klipsch and U.S. Pat. No. 4,308,932 to Keele, Jr.
The Klipsch patent is directed to a radial horn of "astigmatic" construction, wherein expansion of an acoustic signal takes place initially in a single plane before commencing at right angles to that plane. This is desirable to maintain a uniform phase of the signal over the mouth of the horn, such that the wavefront is a substantially spherical surface independent of frequency. The Klipsch device is well suited to circumstances calling for a radial wavefront of constant directivity, but is incapable of generalized coverage control.
The Keele patent discloses an improvement to the Klipsch horn, wherein two opposing side walls are flared outwardly according to a power series formula to enhance low frequency and midrange response. The horn of the Keele patent achieves directional characteristics substantially independent of frequency, but is limited in attainable coverage patterns in the same manner as the Klipsch horn.
Most recently, designers of loudspeaker horns have focused on attaining a uniform direct-field sound pressure level at all listener positions. Uniform sound pressure is difficult to obtain because most listener areas do not match the polar patterns of available loudspeakers. Even when the output of a single source is high enough to cover an area, the source will not suffice if it lacks proper directional characteristics. In addition, the phenomenon of "inverse rolloff", i.e., the decrease in sound pressure with increasing beam area, typically causes pressure to vary drastically over an area covered by a single source. Directivity and rolloff considerations can be addressed with clusters of short, medium and long throw horns directed to different portions of the area, but such systems are significantly more expensive than a single loudspeaker.
Therefore, it is desirable in many applications to provide a horn for directing sound from a single driver over a defined area at substantially constant directivity and pressure level.